There is however one gotcha.
There used to be a very popular product made by Edme called DME. In this product, the D stood for diastatic, and not dry. (It was a syrup). If a recipe calls for this product, the weights will be specified in LME units, and the recipe may require the diastase for starch conversion.

Some recipes call for both liquid and dme can you just use one or the other?

Right, generally they are interchangeable (except for the amounts). I've seen both used in recipes and the reason is that certain canned LME comes in 3.3 pound cans. So, to make up the recipe and not use two cans or a portion of a second can, it was supplemented with DME.

I prefer DME usually over LME, unless the LME is particularly fresh. DME is easier to store and keep on hand and weigh. Plus, I keep DME around for starters.

__________________Broken Leg BreweryGiving beer a leg to stand on since 2006

From the recipes I've seen (e.g. Harpoon IPA from the BYO clone issue), the DME is used for the entire boil and the LME is used as a late addition (last 15 minutes of the boil).

The main benefit is that the LME is often cheaper, at least at my LHBS. If you boil LME it the entire 60 or 90 minutes however, it will caramelize and lead to a slightly darker beer. If you put the LME in as a late addition, it doesn't have as much boil time to caramelize so you don't have to worry about a darker beer.

BYO also also seems to use both LME and DME when the LME comes in a can or other "fixed" size and more extract is needed (e.g. 6.6 lb can or LME and 2 lbs DME).

With all that said, you can generally use either, as long as you end up with the same starting gravity.